Here - PMOD/WRC
Here - PMOD/WRC
Here - PMOD/WRC
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Figure 1. Then new NPL cavity pyroelectric detector.<br />
A total of 35 CPDs have been assembled and studied over<br />
the period from 1998 to 2003. The gain G(λ) [5] as a<br />
function of wavelength was measured for the majority of<br />
these CPDs. Figure 2 shows the spectral responsivity of a<br />
3 mm by 3 mm Pt-black-coated pyroelectric detector<br />
measured in the 0.8 µm to 25 µm wavelength range<br />
relative to both the CPD75 and pyroelectric detector 75 i.e.<br />
against the same pyroelectric detector, with and without a<br />
gold-coated reflective hemisphere. These plots are typical<br />
for the pyroelectric detectors with the very best quality<br />
gold-black coatings. Division of the two traces shown in<br />
Figure 2 gives the relative spectral responsivity of CPD75<br />
relative to pyroelectric detector 75. This ratio is also the<br />
gain G(λ) resulting from the addition of the reflective<br />
hemisphere onto the pyroelectric detector. The values of<br />
G(λ) as a function of wavelength for CPD75 are shown in<br />
Figure 3. Figure 3 shows that the gain is greater than unity<br />
for all wavelengths. This confirms that the gold-black<br />
coating deposited on pyroelectric detector 75 is not perfect<br />
and that the addition of the hemisphere redirects some of<br />
the radiation reflected by the gold-black coating back to<br />
the gold-black coating thus enhancing its effective<br />
absorbance and therefore its response. Moreover Figure 3<br />
shows that G(λ) increases monotonically with increasing<br />
wavelength. This indicates that the reflectance of the gold<br />
black coating gradually increases with wavelength<br />
(assuming the effective reflectance of the reflecting<br />
hemisphere is not a function of wavelength). This<br />
observation is in agreement with measurements of the<br />
reflectance of all good quality gold-black coatings [6].<br />
Relative response<br />
2<br />
1.9<br />
1.8<br />
1.7<br />
1.6<br />
1.5<br />
1.4<br />
T13/cp75<br />
T13/P75<br />
1.3<br />
0 5 10 15 20 25<br />
Wavelength/µm<br />
Figure 2: Spectral response of a 3 mm by 3 mm<br />
Pt-black-coated pyroelectric detector measured in the<br />
0.8 µm to 25 µm wavelength range relative to both CPD75<br />
and pyroelectric detector 75<br />
All parameters of the cavity pyroelectric detectors were<br />
fully evaluated and will be presented at the meeting. These<br />
include their relative and absolute spectral responsivities,<br />
spatial uniformity of response, linearity of response,<br />
temperature coefficient of response, temporal response,<br />
long term stability and noise characteristics.<br />
The presentation will also describe how the relative<br />
spectral responsivity of a cavity pyroelectric detector is<br />
estimated from the knowledge of its gain G(λ) as a<br />
function of wavelength. It will also deal with the<br />
uncertainty budget developed to estimate the uncertainty of<br />
the NPL infrared relative spectral responsivity scale.<br />
Gain<br />
1.045<br />
1.04<br />
1.035<br />
1.03<br />
1.025<br />
1.02<br />
1.015<br />
1.01<br />
1.005<br />
1<br />
0 5 10 15 20 25<br />
Wavelength/µm<br />
Figure 3: Gain in the 0.8 µm to 25 µm wavelength range<br />
measured when a reflective hemisphere was added to<br />
pyroelectric detector 75.<br />
Acknowledgements Special thanks must go to John Lehman of<br />
NIST Boulder for assembling all the pyroelectric detector<br />
modules used in the new NPL CPDs and for many useful<br />
discussions. The author also wishes to thank Nick Nelms of the<br />
Space Research Centre of the University of Leicester, UK and<br />
Mark Proulx of Laser Probe Inc.<br />
Reference<br />
1. G. W. Day, C. A. Hamilton and K. W. Pyatt,<br />
Applied Optics, 15, 1865-1868, 1976.<br />
2. D. H. Nettleton, T. R. Prior and T. H. Ward,<br />
“ Improved spectral responsivity scales at NPL, 400<br />
nm to 20 µm”, Metrologia, 30, 425-432, 1993.<br />
3. L. P. Boivin, “Current work at NRC of Canada on<br />
absolute radiometer based calibrations in the infrared”,<br />
in New developments and Applications in optical<br />
Radiometry, N. P. Fox and D. H. Nettleton (eds), 1989,<br />
81-88.<br />
4. J. Lehman, E. Theocharous, G. Eppeldauer, and C.<br />
Pannel, “Gold-black coatings for freestanding<br />
pyroelectric detectors” Measurement Science and<br />
Technology, 14, 916-922, 2003.<br />
5. E. Theocharous, “The establishment of the NPL<br />
infrared relative spectral responsivity scale using<br />
cavity pyroelectric detectors”, to be published.<br />
6. N. Nelms and J. Dawson, “Goldblack coating for<br />
thermal infrared detectors” Accepted for publication in<br />
Sensors and Actuators, January 2005.<br />
302